1. Field of the Invention
The present invention relates to a rotation speed detecting structure for a bearing that performs relative rotation and, more specifically, relates to a magnetic encoder for detecting the rotation speed and to a bearing unit with the attached magnetic encoder.
2. Description of the Related Art
[Patent Literature 1] Japanese Patent Application laid-open No. 2002-286739
In conventional practice, bearing units for automobile wheels and the like are equipped with a wheel rotation speed detecting apparatus for preventing differences in rotation among a plurality of wheels, and the following type of structure is widely used for this detecting apparatus.
Specifically, a rotation speed detecting apparatus has a pulse generating rotor and a sensor for sensing the pulse, and a rotation speed detector wherein a pulse generating encoder with a multi-pole magnet is disposed in the radial part of a reinforcing ring and wherein a sensor is provided near the pulse generating encoder has been developed and put into practical use as a common structure thereof. In many cases, such a pulse generating encoder or, in other words, a magnetic encoder, is combined with a seal member having seal lips in order to efficiently utilize the narrow space of the bearing unit.
The invention is described in further detail below with reference to the diagrams.
One embodiment of such a conventional example will now be described with the aid of a diagram. As shown in FIG. 4, a bearing unit structure is used in which a multi-pole magnet 3 is attached to a reinforcing ring 2 fitted into an inner wheel 1b to form a magnetic encoder E, and in which the rotation of the multi-pole magnet 3 is sensed and detected with a rotation speed detecting sensor 4. A structure combined with a seal member S is shown here, and the bearings 1a, 1b are protected from the ingress of moisture or foreign matter by the seal lips 5 of the seal member S being in frictional contact with the inner peripheral surface of the reinforcing ring 2.
In the multi-pole magnet 3 of such a magnetic encoder E, however, the N/S poles are subjected to multi-polar magnetization in a circumferential manner, so extreme caution must be taken during handling to prevent contact with magnetized granular materials, parts that have a magnetic force, or the like until the magnet is mounted on the bearings 1a, 1b, and safety must be ensured during storage as well. In addition, inconveniences arise in the sense that the reinforcing rings 6 of other seal members S become magnetized and are difficult to pull out during stacked storage.
Because of such conditions, iron sand and other such foreign matter tend to adhere to the surface of the multi-pole magnet 3 during regular handling, and in the event that foreign matter enters and adheres between the multi-pole magnet 3 and the rotation speed detecting sensor 4, the rotation speed detecting apparatus develops fatal flaws in that the multi-pole magnet 3 is caused to generate signal errors by the foreign matter and is damaged or otherwise affected by the foreign matter biting into the magnet during rotation, creating errors in rotation speed detection.
In view of the above and in order to prevent this situation from occurring, structures are being developed in which a synthetic resin cap or a metal cover (neither are shown in the diagrams) is temporarily attached or crimped on so as to cover the surface of the multi-pole magnet 3, but this approach is disadvantageous in that in the first case, which involves the synthetic resin cap, the multi-pole magnet 3 cannot be protected until the final step because of the need to remove the cap when a knuckle is inserted, and in the second case, which involves the metal cover, numerous manufacturing steps are performed and high costs are entailed.
In view of such drawbacks, it is therefore a first object of the invention to provide a magnetic encoder equipped with a protective apparatus wherein the pulse generator is completely protected and the sensing capabilities and durability are remarkably improved.
It is a second object of the invention to provide a bearing unit with an attached magnetic encoder that is obtained by incorporating the magnetic encoder mentioned above as the present invention, which is equipped with a protective apparatus wherein the pulse generator is completely protected and the sensing capabilities and durability are remarkably improved.